The present apparatus and process relate to coating of a conductor by electrodeposition processes and more particularly to "spark" electrodeposition.
The phenomenon of metal transfer by short duration electric discharge applied repetitively at a rate usually greater than 50 cycle per second has been known for over 200 years. The actual mechanisms involved in material transfer by electrodeposition is subject to speculation. However, it is usually assumed that a gas bubble forms about the spark discharge and persists for a time longer than the spark itself. Metal melted due to the high temperature of the spark is transferred from the electrode to the workpiece surface via the expanding gas bubble. Generally, maximum material transfer is made by moving the electrode over the workpiece and by making the electrode the anode and the workpiece surface the cathode of the discharge circuit. Another reason for providing relative motion between the electrode and workpiece is the need to continually fracture the adhesive junctions forming as the electrical discharges occur and the molten metal deposits and solidifies.
Both electrode and workpiece are conductive and form the terminal poles of a direct current power source. Linear vibration of the electrode is commonly used to provide relative motion between the two poles. It is typically achieved by mounting the electrode to the armature of a solenoid. Typical operation of such a solenoid is 120 cycles per second.
Alternate motions of the electrode have been attempted, including rotation of the electrode about its axis and combinations of rotation and linear vibration to provide intermittent contact with the surface, thereby causing repetitive electrical discharges.
The individual discharges through the electrode must be of short duration (less than 100 microseconds) and the energy level at 0.01 to 4 joules for material transfer to satisfactorily occur. A condition known as arcing occurs when the electrical discharge is of low intensity and long duration. Duration of the spark during arcing can be of a magnitude of one hundred times longer than is desirable for spark deposition purposes. It therefore becomes very desirable to minimize the duration of the spark while maintaining the selected energy level for proper deposition.
Furthermore, mechanical restrictions of vibrating mechanisms used with prior electrode combinations limit the vibrating frequency to about 100 Hz. The combination of long pulse duration and slow pulse frequency has often resulted in unacceptable transfer of electrode material onto the workpiece.
E. H. Thornton and R. G. Davies in an article in "Metals Technology" copyrighted 1979 by the Metals Society, 1 Carlton House, Terrance, London SW1Y5DB discloses a "high-rate spark hardening" of hot-forging dyes in which a fast capacitive discharge is achieved by switching the circuit by a thyristor (SCR) that is fired when a level of charge is detected in an associated discharge capacitor. Resistance in the circuit is kept low to increase the discharge energy. The total charge-discharge cycle occupies a space of approximately 250 microseconds with a spark frequency of about 4 kHz. The results are reported as being more satisfactory than were previously achieved with mechanical solenoid vibration systems. The article discloses an application method by which the electrode is rotated at a speed about 100 revolutions per minute to avoid "sticking" of the electrode to the work surface. A commutator provides electrical contact between the discharge circuitry and the rotating electrode. Although resistance through the commutator and "built-in" resistances in the circuitry occurred, an application rate increase to approximately nine square centimeters per minutes was obtained.
U.S. Pat. No. 3,097,291 to Adcock discloses a method for depositing a hard metal such as carbide on a workpiece such as a shaping tool through electric sparking. Adcock provides a capacitive discharge circuitry designed to discharge through a hard metal anode in contact with the conductive workpiece. The discharge is controlled by one or more electronic switches such as thyratrons, that are cyclically charged to uniform energy levels. When the thyratrons become conductive, the capacitors will discharge through them and through primary windings of a transformer. This causes discharge between the sparking anode and the workpiece. Specific disclosure is made of a three phase circuitry so each capacitor is charged fully before cyclic discharge. Again, a rotating or sliding electrode is disclosed for applying the discharges. Additionally, resistance is placed in the circuitry, presumably for the purpose of charging the capacitors. The rate of pulsation is stated to be at 100 pulses per second.
U.S. Pat. No. 3,316,381 to W. W. Gibson discloses a power supply and method for metal surfacing. Gibson discloses use of a three phase circuit using a single capacitor in series feeding an inductor. Secondary winding of the inductor is connected to an electrode. Gibson does not disclose a switching circuit per se of the capacitive discharge type but, rather, a capacitive-conductive oscillator circuit to produce electrical resonance for opening and closing the circuit. The electrode is held some distance from the workpiece to produce an open spark gap. This gap can be varied to correspondingly vary the characteristics of the surfacing. However, the electrode cannot be placed constantly in direct contact with the electrode due to the nature of the circuit. Lack of resistance between the power supply voltage and capacitor or a positive, "off-on" switch between the capacitor and electrode results in "long time" arc durations that are possibly acceptable for heavy duty surfacing applications resembling arc welding, but cannot be made workable in smaller scale operations.
British Pat. No. 756,727 to Rudorff discloses improvements relating to working electrically conductive materials by electric erosion. Here, the capacitive discharge circuitry makes use of diodes to avoid intermittent cyclical reversal of current. A somewhat similar circuit arrangement is illustrated in U.S. Pat. No. 3,036,197 to M. Bruma. Bruma, however, is primarily concerned with an apparatus for protecting the associated components of the device in a short circuit situation.
U.S. Pat. No. 3,277,266 to Blaszkowski discloses apparatus for hard coating metal surfaces. This device is intended to accurately support an electrode with controlled contact pressure as it moves over the surface being treated. Provisions are also made for both vibrating and rotating the electrode during discharge deposition. In fact, it is maintained that simultaneous vibration and rotation produce a superior surface finish.
U.S. Pat. No. 3,277,267 to Blaszkowski discloses a method and apparatus for treating electrically conductive surfaces. This disclosure deals with various types of apparatus for treating conductive surfaces by intense localized heating. Blaszkowski discusses the value of utilizing a capacitive discharge circuit to avoid "arcing" and for extinguishing the arc when the electrode is pulled from contact with the workpiece. With such circuitry, the electrode will discharge across the point of contact rather than across a gap as in arc welding. Another stated advantage is that heat is not generated continuously. The patent, however, deals primarily with the sliding motion of the electrode parallel to the workpiece. Discharge is accomplished by contact at individual small "high points" along the workpiece surface. It can therefore be theorized that only the high points along the workpiece surface receive the deposits of electrode material. Furthermore, the workpiece surface must necessarily be rough in order for the "high points" to exist. Frequency of discharge is varied by altering the rotational speed or sliding motion of the electrode. The capacitor is said to recharge as the contact region shifts from one point to another. Current flow discharge time is stated to be from 15 to 40 microseconds. A variable resistor permits regulation of the discharge current to the electrode. Furthermore, the discharge time has a substantial effect on the charge. Blaszkowski discusses several electrical and mechanical means by which the circuit is discharged other than by "point-to-point" contact between the electrode and workpiece. Among the various other devices, Blaszkowski discloses the use of a thyratron tube or controlled rectifiers operating within a range of 200 to 50,000 cycles per second. Such switching allows the capacitors to recharge without discharge current flow that can otherwise occur. Mechanical switching, however, using a cam braker system is preferred.
U.S. Pat. No. 3,617,680 to Grosskopf discloses a circuit utilizing thyristors to shunt current away from a spark gap when a short circuit is detected. A thyristor is also used in series with a capacitor to produce a current in the spark gap that opposes the short circuit current. The circuitry here is used in a process for removing material from a workpiece rather than depositing the material from the electrode.
U.S. Pat. No. 3,098,150 to Inoue discloses a spark discharge metal depositing apparatus using a vibrating electrode mechanism that is energized by the same pulse that discharges the condensor. Brush or commutator contact with the electrode is shown in FIG. 10 of the drawings. Isolation of power sources is also shown to provide power to drive the vibrator and sparking electrode semi-independently. One of the objects of the Inoue device is to provide an apparatus that is automatically self-regulating without requiring mechanical-electrical switching devices in order to control flow of current. Inoue emphasizes that no other switching mechanism is required.
U.S. Pat. No. 3,969,601 to Rocklin discloses a machine for performing eroding or treating functions on a metal workpiece. The metal treating circuitry of this device includes the typical resistance-capacitance (RC) circuit for spark generation in the metal treatment mode. Load sensing apparatus is used to determine feed rate and spark quality.
U.S. Pat. No. 3,524,596 again to Rocklin discloses a device for depositing a thin layer of hard conductive material such as tungsten carbide onto a conductive surface. A mechanism is provided to allow interchanging of electrodes and operation as a "percussion" welder in which the electrode is driven forcibly against the workpiece. The electrode is then withdrawn quickly to break electrical contact so that a portion of the electrode material will be pulled away from the electrode and deposited on the workpiece surface. The usual RC circuit is involved for producing the high current discharge.
The Rocklin device is adapted for use with a hand-held electrode "gun". Other U.S. patents showing similar vibrating or rotating hand-held electrode deposition apparatus are: 3,763,343 to Rocklin; 3,614,373 to Skilling; 3,415,971 to Shaffer; and 3,415,970 to Cline. All disclose RC or LC (inductance-capacitance) circuits for producing the current pulsations.
The circuitry of my invention makes use of a thyristor for controlling discharge of a capacitor in a low resistance discharge circuit. The thyristor is capable of discharging the capacitor through a positively biased electrode, depositing a minute amount of material on the negatively biased surface of the other conductor. The thyristor is triggered or made to conduct by an independent source, producing trigger pulses. The thyristor will turn off or stop conducting when current direction reverses in the circuit due to inherent inductance and capacitance (LC). By using the thyristor and reducing current reversal within the circuit, I am able to eliminate need for a ballast resistor in the circuit that would be otherwise required for charging of the storage capacitor. The electrode in the current apparatus is oscillated about a central axis to provide moving contact with the workpiece. The oscillating electrode can be moved over the workpiece surface in any desired direction to deposit the electrode material onto the workpiece surface. Axial oscillation of the electrode, coupled with the rapid capacitance discharge capability of the circuit, results in a quality coating of the workpiece without requiring accurate electrode positioning control or maintaining a specific workpiece-electrode gap.